Post-pulse eliminator



y 2 1958 M. HOBBS 2,845,537

POST-PULSE ELIMINATOR Filed Jan. 23, 1956 Fig. I.

I 2O 22 FIG D c Z Modulator a 5- Magnetron Voltage Across windings 20 and 22 Without Reoco a b tor 24 g g c A V 'V Time 3. Voltage Across windings 20 0 b Fig and 22 With The Addition Of Reactor 24 Fig. 4.

Current Through Reactor 24 WITNESSES INVENTOR (3 3 KG; William M. Hobbs United States Patent POST-PULSE ELIMINATOR William M. Hobbs, Brooklyn Park, Md., assignor to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Application January 23, 1956, Serial No. 560,482

3 Claims. (Cl. 250-36) oscillations, called post-pulse oscillations, are. frequently of suflicient amplitude to refire the magnetron subsequent to the main pulse from the generator. The oscillations are due to the switching action of the magnetron which fails to continue conduction as the voltage across it decays below its firing level. This switching action of the magnetron causes incomplete discharge of the energy in the pulse forming network of the pulse generator. In addition to the energy left over in the pulse forming network, some energy is also stored in the pulse transformer which couples the generator to the magnetron. In conventional installations, the left over energy is dissipated by the oscillations which take place, the total time for dissipation being somewhat long due to the relatively high Q and low resonant frequency of the components in the circuit.

It is an object of this invention to provide means for eliminating post-pulse oscillations associated with a magnetron or similar device driven by a pulse generator. As will become apparent from the following detailed description, the magnetron is coupled through a transformer to a pulse generator. In accordance with the present invention, a saturable reactor is connected in parallel with one of the windings of the transformer. By applying a bias voltage of correct magnitude to this reactor, it can be made to saturate after a predetermined amount of time has elapsed subsequent to the initiation of a pulse by the pulse generator. With this arrangement, any postpulse oscillations will be shunted around the transformer winding whereby none of the oscillations can be applied to the magnetron to cause it to refire.

Another object of the invention lies in the provision of a post-pulse eliminator which is simple and rugged in construction.

Further objects and features of the invention will become apparent from the following detailed description taken in connection with the accompanying drawings which form a part of this specification and in which:

Figure 1 is a schematic circuit diagram of one embodiment of the invention;

Fig. 2 is a waveform illustrating the post-pulses caused by the switching action of a magnetron without the present invention;

Fig. 3 is a waveform of the voltage appearing across the pulse transformer of Fig. 1, illustrating the elimination of post-pulse oscillations by use of the present invention; and

Fig. 4 is a waveform showing the current fiow through the saturable reactor of Fig. 1.

Referring to Figure 1, the circuit shown includes a pulse generator or modulator .10, shown in block form. Any pulse generator may be usedwith the present invention which will supply a series of output voltagev pulses, appearing across outputterminals 12 and 14. The pulse generator supplies its output pulses to a magnetron 16, also shown in block form. A:pulse transformer 18, having primary and secondary windings 20 and .22, respectively, couples generator 10 to a magnetron 16. In shunt with primary winding 20 is a saturable reactor 24 having a core member 26. The core26 of reactor 24 is formed from rectangular hysteresisloopmaterial. This material, well known to thoseskilled in; the art,has a sharp cutoff point between conditionsof saturation and unsaturation so that the impedance of the transformer windings can change almost instantaneously froma relatively high value to almost zero. A bias winding 28 for reactor 24 is connected through a voltage divider 30 to a source of direct current bias voltage 32.

If the saturable reactor 24 were notincluded in the circuit of Figure 1, the voltage waveform across transformer 18 would appear as the waveform of Figure 2. From point a to point 0 on the waveform,energy isstored in the pulse transformer 18. When this energy is released, the succeeding oscillations are produced, and if these oscillations are of sufficient amplitude they will cause the magnetron to refire before the next voltage pulse from the generator 10.

In order to eliminate these oscillations, the saturable reactor 24 is added. To obtain a full understanding of the improved circuit of the invention, an examination of the induced voltage equation for a reactor should be made. This equation is:

where B=flux density of the core in webbers per square unit of area;

k=constant determined by the core material;

N=number of turns of wire in the winding of the inductor;

A=cross sectional area of the core of the inductor;

eainstantaneous voltage; and

t=time in seconds.

When the reactor has a core formed from rectangular hysteresis loop material, as in the present case, it will saturate and present a very low impedance when the flux density B reaches a predetermined value. Since the time required for the flux density to reach saturation is a function of applied voltage, the time required for reactor 24 to saturate can be varied by altering the bias voltage applied to Winding 28.

In the present case the bias voltage on winding 28 is adjusted by voltage divider 30 so that reactor 24 will saturate at point b on the waveform of Figure 2. Thus, Winding 20 is short circuited at point b because of the very low impedance which reactor 24 now presents, and the waveform of Figure 3 appears across the windings of transformer 18. The rapid reversal of voltage across reactor 24 after it saturates (as shown in Figure 3) due to its high frequency resonance with the circuit capacitances, provides the necessary value of the factor fedt, measured in volt-seconds, to bring the reactor back to its original unsaturated condition. The fact that the current surges to a very large value through reactor 24 immediately following a pulse (Figure 4) gives rise to a fast dissipation of the left over energy by causing large energy losses in the reactor. These losses, of course, can be varied by the design of reactor 24.

Since there is some variation in left over energy and in the pulse width from unit to unit due to manufacturing vide for a simple technique in adjusting for zero postpulse in production alignment of units.

Although the invention has been described in connection with a certain specific embodiment, it should be readily apparent to those skilled in the art that various changes in form and arrangement of parts can be made 'to suit requirements without departing from the spirit and scope of the invention.

I claim as my invention: 1. In combination with a pulse generator adapted to supply energy pulses 'of predetermined pulse duration to a magnetron or other similar device, a pulse transformer coupling the output of said generator to said magnetron,

primary and secondary windings for said transformer, a

saturable inductor connected in parallel with said primary winding, and means for applying a bias voltage to said inductor, said bias voltage being of a magnitude such that 'the inductor will saturate and short'circuit said primary winding after an amount of time equal to said predetermined pulse duration has elapsed subsequent to the initiation of a pulse supplied by said generator.

2. In combination with a pulse generator adapted to supply energy pulses of predetermined pulse duration to a magnetron or other similar device, a pulse transformer coupling the output of said generator to said magnetron, primary and secondary windings for said transformer, and a saturable inductive device connected in parallel with said primary winding for short circuiting said primary winding after an amount of time equal to said predetermined pulse duration has elapsed subsequent to the initiation of a pulse supplied by said generator.

3. In combination with a source of voltage pulses each of which has a predetermined pulse duration, a pulse transformer having its primary winding connected to said source of voltage pulses, a saturable inductive device connected in shunt with said primary winding, and a source of bias voltage inductively coupled to said inductive device, said source of bias voltage being of a magnitude such that the inductive device will saturate and short circuit said primary winding after an amount of time equal to said predetermined pulse duration has elapsed subsequent to the initiation of a pulse from said source of pulses.

References Cited in the file of this patent U I v UNITED STATES PATENTS 2,727,160 Sunderlin Dec. 13, 1955 

